Shawn Joseph Marshall

Southeastern Alberta exhibits persistent spring cooling (−0.29°C decade−¹ since 1980) and winter warming stasis – a rare multi-decadal cold anomaly that contrasts with broader North American warmin...

Quantifying the total liquid water amounts (LWAs) in the Greenland ice sheet (GrIS) is critical for understanding GrIS firn processes, mass balance, and global sea level rise. Although satellite microwave observations are very sensitive to ice sheet melt and thus can provide a way of monitoring the ice sheet melt globally, estimating total LWA, especially the subsurface LWA, remains a challenge. Here, we present a microwave retrieval of LWA over Greenland using enhanced-resolution L-band brightness temperature (T_B) data products from the National Aeronautics and Space Administration (NASA) Soil Moisture Active Passive (SMAP) satellite for the 2015–2023 period. L-band signals receive emission contributions deep in the ice sheet and are sensitive to the liquid water content (LWC) in the firn column. Therefore, they can estimate the surface-to-subsurface LWA, unlike higher-frequency signals (e.g., 18 and 37 GHz bands), which are limited to the top few centimeters of the surface snow during the melt. We used vertically polarized T_B (V-pol T_B) with empirically derived thresholds to detect liquid water and identify distinct ice sheet zones. A forward model based on radiative transfer (RT) in the ice sheet was used to simulate T_B. The simulated T_B was then used in an inversion algorithm to estimate LWA. Finally, the retrievals were compared with the LWA obtained from two sources. The first source was a locally calibrated ice sheet energy and mass balance (EMB) model, and the second source was the Glacier Energy and Mass Balance (GEMB) model within NASA's Ice-sheet and Sea-level System Model (ISSM). Both models were forced by in situ measurements from six automatic weather stations (AWSs) of the Programme for Monitoring of the Greenland Ice Sheet (PROMICE) and the Greenland Climate Network (GC-Net) located in the percolation zone of the GrIS. The retrievals show generally good agreement with both the references, demonstrating the potential for advancing our understanding of ice sheet physical processes to better project Greenland's contribution to the global sea level rise in response to the warming climate.

Over the period 2021–2024, glaciers in Western Canada and the conterminous US (WCAN‐US), and Switzerland respectively lost mass at rates of 22.2 ± 9.0 and 1.5 ± 0.3 Gt yr ⁻¹ representing a twofold increase in mass loss compared to the period 2010–2020. Since 2020, total ice volume was depleted by 12% (WCAN‐US) and 13% (Switzerland). Meteorological conditions that favored high rates of mass loss included low winter snow accumulation, early‐season heat waves, and prolonged warm, dry conditions. High transient snow lines, and impurity loading due to wildfires (WCAN‐US) or Saharan dust (Switzerland) darkened glaciers and thereby increased mass loss via greater absorbed shortwave radiation available for melt. This ice‐albedo feedback will lead to continued high rates of thinning unless recently exposed dark ice and firn at high elevations is buried by seasonal snowfall. Physical models that simulate impurity deposition and movement through firn and ice are needed to improve future projections of glacier mass change.

Surface melting and consequent runoff/refreezing play an increasingly crucial role in the Greenland Ice Sheet (GrIS) Surface Mass Balance (SMB) and its contribution to the global sea-level rise. Space-based L-band radiometry offers a promising tool for quantifying the total surface-to-subsurface liquid water amount (LWA) in the firn, in addition to providing the areal extent and duration of seasonal surface snow melt. Here, we evaluate the performance of commonly used microwave dielectric mixing models in determining the total LWA using a snow microwave emission and radiative transfer model in conjunction with L-band (1.4 GHz) brightness temperature (TB) observations from Soil Moisture and Ocean Salinity (SMOS) and Soil Moisture Active Passive (SMAP) missions. The L-band TB responds to the real and imaginary parts of the firn dielectric constant, which increases markedly with liquid water content (LWC) in the firn. The measured dielectric constant is translated into LWA using a model between snow LWC and the dielectric constant. The formulation of the effective dielectric constant of the ice, air, and water mixture is key to accurately quantifying LWA; as it is independent of the radiometer measurement, it adds an uncertainty component to the LWA retrieval that is solely depending on the accuracy of this dielectric mixing model. We apply different dielectric mixing formulations in the forward model to estimate LWA, which we compare to the corresponding LWA from a locally calibrated ice sheet Energy and Mass Balance (EMB) model and the Glacier Energy and Mass Balance (GEMB) model within NASA’s Icesheet and Sea-Level System Model (ISSM). The EMB model was driven by in situ measurements from automatic weather stations (AWS) of the Programme for Monitoring of the Greenland Ice Sheet (PROMICE) and Greenland Climate Network (GC-Net) located in the percolation zone of the GrIS, and the GEMB model was forced with the ERA-5 reanalysis products. Both models were initialized with relevant in situ profiles of density, snow and firn stratigraphy, and the sub-surface temperature measured at the AWS locations. The agreements and discrepancies between the LWA estimates from the mixing models and their comparison with the LWA from firn models will be presented. The analysis assesses the impact of the dielectric mixing model choice on the LWA retrieval algorithm to create an observational dataset of seasonal LWA across GrIS.

Quantifying the total liquid water amounts (LWA) in the Greenland ice sheet (GrIS) is critical for understanding GrIS firn processes, mass balance, and global sea-level rise. Although satellite microwave observations are very sensitive to ice sheet melt and thus can provide a way of monitoring the ice sheet melt globally, estimating total LWA, especially the sub-surface LWA, remains a challenge. Here, we present a microwave retrieval of LWA over Greenland using enhanced resolution L-band brightness temperature (TB) data products from the NASA Soil Moisture Active Passive (SMAP) satellite for the 2015–2023 period. L-band signals receive emission contributions deep in the ice sheet and are sensitive to the liquid water content (LWC) in the firn column. Therefore, they can estimate the surface-to-subsurface LWA, unlike higher frequency signals (e.g., 18 and 37 GHz bands), which are limited to the top few centimeters of the surface snow during the melt. We used vertically polarized TB (V-pol TB) with empirically derived thresholds to detect liquid water and identify distinct ice sheet zones. A forward model based on radiative transfer in the ice sheet was used to simulate TB. The simulated TB was then used in an inversion algorithm to estimate LWA. Finally, the retrievals were compared with the LWA obtained from two sources. The first source was a locally calibrated ice sheet energy and mass balance (EMB) model, which was forced by in situ measurements from six automatic weather stations (AWS) of the Programme for Monitoring of the Greenland Ice Sheet (PROMICE) and Greenland Climate Network (GC-Net) located in the percolation zone of the GrIS. The second source was the corresponding LWA obtained from the Glacier Energy and Mass Balance (GEMB) model within the National Aeronautics and Space Administration’s (NASA) Ice-sheet and Sea-Level System Model (ISSM). The retrievals show generally good agreement with both the references, demonstrating the potential for advancing our understanding of ice sheet physical processes to better project Greenland’s contribution to the global sea level rise in response to the warming climate.

Quantifying the total liquid water amounts (LWA) in the Greenland ice sheet (GrIS) is critical for understanding GrIS firn processes, mass balance, and global sea-level rise. Although satellite microwave observations are very sensitive to ice sheet melt and thus can provide a way of monitoring the ice sheet melt globally, estimating total LWA, especially the sub-surface LWA, remains a challenge. Here, we present a microwave retrieval of LWA over Greenland using enhanced resolution L-band brightness temperature (TB) data products from the NASA Soil Moisture Active Passive (SMAP) satellite for the 2015–2023 period. L-band signals receive emission contributions deep in the ice sheet and are sensitive to the liquid water content (LWC) in the firn column. Therefore, they can estimate the surface-to-subsurface LWA, unlike higher frequency signals (e.g., 18 and 37 GHz bands), which are limited to the top few centimeters of the surface snow during the melt. We used vertically polarized TB (V-pol TB) with empirically derived thresholds to detect liquid water and identify distinct ice sheet zones. A forward model based on radiative transfer in the ice sheet was used to simulate TB. The simulated TB was then used in an inversion algorithm to estimate LWA. Finally, the retrievals were compared with the LWA obtained from two sources. The first source was a locally calibrated ice sheet energy and mass balance (EMB) model, which was forced by in situ measurements from six automatic weather stations (AWS) of the Programme for Monitoring of the Greenland Ice Sheet (PROMICE) and Greenland Climate Network (GC-Net) located in the percolation zone of the GrIS. The second source was the corresponding LWA obtained from the Glacier Energy and Mass Balance (GEMB) model within the National Aeronautics and Space Administration’s (NASA) Ice-sheet and Sea-Level System Model (ISSM). The retrievals show generally good agreement with both the references, demonstrating the potential for advancing our understanding of ice sheet physical processes to better project Greenland’s contribution to the global sea level rise in response to the warming climate.

Quantifying the total liquid water amounts (LWA) in the Greenland ice sheet (GrIS) is critical for understanding GrIS firn processes, mass balance, and global sea-level rise. Although satellite microwave observations are very sensitive to ice sheet melt and thus can provide a way of monitoring the ice sheet melt globally, estimating total LWA, especially the sub-surface LWA, remains a challenge. Here, we present a microwave retrieval of LWA over Greenland using enhanced resolution L-band brightness temperature (TB) data products from the NASA Soil Moisture Active Passive (SMAP) satellite for the 2015–2023 period. L-band signals receive emission contributions deep in the ice sheet and are sensitive to the liquid water content (LWC) in the firn column. Therefore, they can estimate the surface-to-subsurface LWA, unlike higher frequency signals (e.g., 18 and 37 GHz bands), which are limited to the top few centimeters of the surface snow during the melt. We used vertically polarized TB (V-pol TB) with empirically derived thresholds to detect liquid water and identify distinct ice sheet zones. A forward model based on radiative transfer in the ice sheet was used to simulate TB. The simulated TB was then used in an inversion algorithm to estimate LWA. Finally, the retrievals were compared with the LWA obtained from two sources. The first source was a locally calibrated ice sheet energy and mass balance (EMB) model, which was forced by in situ measurements from six automatic weather stations (AWS) of the Programme for Monitoring of the Greenland Ice Sheet (PROMICE) and Greenland Climate Network (GC-Net) located in the percolation zone of the GrIS. The second source was the corresponding LWA obtained from the Glacier Energy and Mass Balance (GEMB) model within the National Aeronautics and Space Administration’s (NASA) Ice-sheet and Sea-Level System Model (ISSM). The retrievals show generally good agreement with both the references, demonstrating the potential for advancing our understanding of ice sheet physical processes to better project Greenland’s contribution to the global sea level rise in response to the warming climate.

With the growing concern of climate change, an accurate estimation of the total meltwater amounts (MWA) in the Greenland ice sheet (GIS) becomes crucial for understanding the physical processes of the GrIS and its mass balance, thereby enabling accurate prediction of its contribution to the global sea-level rise. Satellite microwave radiometers have been widely used for monitoring ice sheet melting for the last four decades; nevertheless, quantification of total MWA, especially the sub-surface MWA, remains a challenge.Here, we used the enhanced resolution L-band brightness temperature (TB) observations from the NASA Soil Moisture Active Passive (SMAP) mission to quantify the magnitude of the total MWA in GrIS for 2015-2023. Because of the larger penetration depth, L-band signals can track liquid water in deeper layers and provide a reliable estimate of surface-to-subsurface MWA, contrary to the higher frequency signals (18 or 37 GHz bands), which are limited to the top few centimeters of the surface snow. The algorithm uses vertically polarized (V-pol) TBs and an empirically derived adaptive thresholding technique to detect melt events. A simple microwave emission model, based on ice sheet radiative transfer, was used to simulate L-band TBs. The simulated TBs were then used in an inversion algorithm for MWA retrieval.Finally, the retrieval was compared with the corresponding MWA derived from an ice sheet energy and mass balance (EMB) model which was forced by hourly in situ observations from the Programme for Monitoring of the Greenland Ice Sheet (PROMICE) automatic weather station (AWS) network. The model was initialized and constrained by the relevant ice core density and sub-surface temperature profiles. The retrievals generally demonstrate a stronger agreement with the in situ observations in the percolation zone than in the ablation and upper elevation regions. The radiometric sensitivity, meltwater process, and their spatiotemporal variability were analyzed. The results demonstrate the potential for advancing our understanding of ice sheet physical processes to better project Greenland’s contribution to global sea level rise in response to the warming climate.

Successful adaptation and mitigation of rising sea level demands improved constraints on emerging ice sheet processes controlling the magnitude and rate of sea level change in a warming climate. Therefore, it is vital to enhance the confidence in quantitative assessments of present-day ice sheet mass balance arising from meltwater generation and refine the explicit treatment of meltwater refreezing in firn densification models to evaluate the time evolution of runoff/retention and surface elevation change. The European Space Agency's Copernicus Imaging Microwave Radiometer (CIMR), scheduled for launch in 2028, responds to this need by measuring the brightness temperature (TB) at 1.4, 6.9, 10.7, 18.7, and 36.5 GHz frequencies multiple times a day over polar regions without gaps at the poles. These measurements can resolve the stratification of the seasonal meltwater from the immediate surface to the deeper firn layers. Furthermore, the 1.4 GHz TB is sensitive to the amount of meltwater.We are developing retrieval algorithms using 1.4 GHz measurements from NASA Soil Moisture Active Passive (SMAP) and ESA Soil Moisture Ocean Salinity (SMOS) satellites and 6.9, 10.7, 18.7, and 36.5 GHz measurements from JAXA Advanced Microwave Scanning Radiometer – EOS (AMSR-E) and AMSR2 instruments on NASA Aqua and JAXA (Global Change Observation Mission – Water) GCOM-W satellites. In the algorithm development, we use ground measurements and coupled energy and mass balance models to test, calibrate, and validate the algorithm. The ground measurements include surface and subsurface measurements from PROMICE/GC-Net (Greenland) and other station and campaign data sets. The model suite includes locally calibrated and forced energy balance models and regionally forced models, such as the Ice Sheet System Model's Glacier Energy and Mass Balance module. The retrieved data product will provide twice-daily parameters such as meltwater amount, melt layer depth, and near-surface snow status (wet/dry) profile. It will also include seasonal parameters such as firn aquifer extent and evolution.The meltwater algorithm for the CIMR mission will eventually encompass two satellites capable of monitoring diurnal melt-freeze cycles and perennial firn aquifers for at least a 15-year mission period. CIMR will measure all frequencies simultaneously, which will eliminate the uncertainties related to different observation times of the current instrument combinations, increase the revisit time of the L-band observations, and improve the spatial resolution of the 6.9, 10.7, 18.9, and 36.5 GHz channels compared to what is currently available.

Glaciers in western North America (WNA) and Switzerland represent important sources of freshwater, especially during times of drought. We employ extensive airborne laser altimetry campaigns in WNA coupled with in-situ surface mass balance measurements for both regions to quantify recent mass change. Over the last three years glaciers within these regions respectively lost mass at rates of -22.8±7.4 and -1.7±0.3 Gt yr-1 which, for both regions, represents an approximate twofold increase in mass loss compared to the period 2010-2020. Based on the estimated glacier volume in the year 2020, total volume change in these regions was depleted by 9% (WNA) and 10% (Switzerland) over the last three years. The year 2023 represents the year of greatest common mass loss for both regions where glaciers in both WNA and Switzerland respectively lost -37.1±10.4 and -1.8±0.3 Gt yr-1. Meteorological conditions that favored high rates of mass loss included low winter snow accumulation, early-season heat waves, and prolonged warm, dry conditions. Loss of firn, high transient snow lines, and potential impurity loading due to wildfires (WNA) or Saharan dust (Switzerland) darkened glaciers and thereby accelerated melt via an increase in absorbed shortwave radiation available for melt. This ice-albedo feedback will lead to continued accelerated loss unless recently exposed dark firn and ice at high elevation can be buried by subsequent snowfall. Rates of mass loss for the years 2021-2023 exceed even those projected for unabated global emissions though the twenty-first century, signaling the need to rapidly mitigate greenhouse gas emissions if glaciers in both regions are to survive.